A friend of mine related today, as shocked as I was to hear it, that he heard that every single study that had ever been conducted on second hand smoke effects was flawed. While I was fairly shocked to hear this (if cigarette smoke causes cancer, why would second hand cigarette smoke not cause cancer?), apparently there's others who think this.

"Secondhand smoke kills, according to a new study!," is frantically shouted from the bold type headlines of newspapers and magazine articles. Mostly it is uneducated and/or issue biased newspaper reporters who add the hype to the already deceitful study. Either they don't bother to read the study for themselves (or they do and don't understand the scientific implications), relying on misleading statements from the researchers themselves, or they are far more interested in the fear mongering factor that makes for good reading and do not bother to check with anyone how accurate or reliable a study is. Not to mention it is more politically correct to nod their head yes and insert their own views and conclusions based on the scary sounding research title and bits of information that they have heard.

Whether it is a reporter with no concept of what it is they are actually reporting, simply relaying the study du jour, or a health organization or researcher submitting a report, none of what they say can be taken at face value. News reports are manipulated and then hyped, as are practically all of the studies on secondhand smoke.

The site appears to go through about 10 various studies on second hand smoke, proceeding to illustrate why they don't show what they intend to show: that second hand smoke, does, indeed, cause cancer.

For once and for all: Is there current good, non-flawed evidence from studies showing that second-hand smoke does cause cancer?

3 Answers
3

When there is doubt about individual studies, the place to go is a meta-study - one that looks at many different experiments, evaluates the quality according to a strict methodology, and pools the results to get stronger statistical findings.

It extracted 55 high quality studies from a pool of 101 that looked at never-smoking women with smoking spouses.

They found an increased risk of lung cancer, across each continent, with no evidence of publication bias.

The abundance of evidence, consistency of finding across continent and study type, dose-response relationship and biological plausibility, overwhelmingly support the existence of a causal relationship between passive smoking and lung cancer.

But wait, that's only in the home. Why would we ban tobacco in the work-place? Oh, because there is another meta-analysis that shows the effect can be even stronger there!

Any chance you can add some more info to your answer, such as average duration and frequency of exposure?
– Sonny OrdellFeb 29 '12 at 6:25

Sorry, @Sonny, I can't give you straight answer there. With 77 different studies, those answers don't appear in the abstracts. However, both meta-studies found the more the exposure, the higher the increase in relative risk.
– Oddthinking♦Feb 29 '12 at 7:27

… the debate is over. The science is clear: secondhand smoke is … a serious health hazard that causes premature death and disease in children and nonsmoking adults.

[…]

Since [the 1986 Surgeon General’s Report] was published, hundreds of peer-reviewed studies and several additional major reports on the health effects of secondhand smoke have been published, and the evidence on these health effects has become even stronger.

There is NO risk-free level of secondhand smoke exposure, with even brief exposure adversely affecting the cardiovascular and respiratory system.

It’s worth pointing out that the report is uncharacteristically clear and drastic. No allowance is made for reasonable doubt.

It’s also worth noting that the source cited in the question disqualifies itself with its name-calling, scientific ignorance (such as reference to “the mysterious SIDS” – there’s nothing at all mysterious about it) and by its routine use of Godwin’s law.

Do you have any info on length and frequency of exposure before a negative impact is observed? I think this is important, as there is (I would imagine) a clear difference between walking past and inadvertently inhaling part some second hand smoke outside and being in a closed room full of smokers for an hour.
– Sonny OrdellFeb 29 '12 at 11:03

@Sonny “before negative impact is observed” is a flawed starting point. As the SG observes, “there is no risk-free level of exposure”. A “safe dose” is a flawed concept. Every single exposure increases the risk and it’s simply impossible to establish a point at which the risk becomes measurable over random background noise. But since it’s a sliding scale, no exposure is safer than one exposure, it’s as simple as that. Even a single exposure to secondhand smoke can potentially cause averse effects. In fact, short-term effects are always observable.
– Konrad RudolphFeb 29 '12 at 11:12

The question is specifically about cancer. I'm skeptical that infrequent short-lived exposure to second hand smoke can significantly or even substantially increase the risk of cancer. I'm not disputing the findings of the studies above, however I think they are less useful without information about the duration and frequency of exposure. Also, "increases the risk" is vague enough to not be useful without specifying how much the risk is increased.
– Sonny OrdellFeb 29 '12 at 11:30

1

My point is that there are huge differences in exposure to second hand smoke and it isn't clear that very minimal exposure can significantly increase cancer risk. I know that many smokers dismiss the risk of second hand smoke outside in an open environment. It would be nice to have something more specific to point to.
– Sonny OrdellFeb 29 '12 at 11:31

1

“The question is specifically about cancer” – only the title, and that should be changed, since the claim is actually about all harm. I cannot comment on the increased cancer risk. It almost certainly exists but I don’t know of studies quantifying it. Cancer risk in general is very hard to quantify, and I assert that your demand is a bit unreasonable in that regard: such data doesn’t exist for most conditions where there is still a clear risk. Even for ionising radiation, where we have hard dosage data (the best data of any risk factor AFAIK), incidence rates cannot be reliably deduced.
– Konrad RudolphFeb 29 '12 at 11:42

Despite the consensus, there are good quality studies that don't find a strong link

I think the scientific consensus is clear: almost everyone thinks secondhand smoke is bad. But skeptics should carefully consider, especially in areas where emotions run as high as this, the alternative views.

Here is a large scale study of the long term effects of secondhand smoke that finds little impact on all cause mortality. The BMJ paper is here. The conclusion:

Conclusions. The results do not support a causal
relation between environmental tobacco smoke and
tobacco related mortality, although they do not rule
out a small effect. The association between exposure
to environmental tobacco smoke and coronary heart
disease and lung cancer may be considerably weaker
than generally believed.

For the sake of balance it is worth reading the follow-up debate. My reading is that there are few direct criticisms of the science but many attacks on the scientists (who have some relationship with the "evil" tobacco industry). The intemperate nature of many responses suggest that it is becoming hard to conduct and publish any research on the topic that does not agree with the consensus. I'd recommend any skeptic to read the paper and then the responses and make their own mind up while trying to consider only criticism of the results and not their funding.

Both poor quality studies agreeing with the consensus and entirely unreferenced claims are widely repeated to justify legislation. One example is the (entirely unreferenced) claim that secondhand smoke kills 600,000 people a year. The claim comes from a WHO factsheet which does not reference any sources. Yet this is repeated as an accurate assessment even by people who should know better such as David Nutt (it is in his, mostly sensible, book *Drugs-without the hot air"). This is not a small number of deaths and, if the claim were true, we would expect strong epidemiological evidence. This is not obviously there in the literature.

Update
There has been some new debate on the subject since the original answer was written.

One author at reasons.com argues that it is becoming easier to challenge the evidence as the political significance of the position you take is now less significant since many countries now have widely accepted indoor smoking bans. The debate is summarised thus (my emphasis):

Several years ago I was talking to an epidemiologist who is skeptical of the idea that smokers pose a mortal threat to people in their vicinity. Although he supported workplace smoking bans, he was frustrated by the willingness of so many anti-tobacco activists and public health officials to overlook or minimize the weakness of the scientific case that secondhand smoke causes fatal illnesses such as lung cancer and heart disease. He wondered when it would be possible to have a calm, rational discussion of the issue, one in which skeptics would not be automatically dismissed as tools of the tobacco industry.

The opinion piece references a new recent study that doesn't suffer from many of the biases that affect many other studies. The bias is explained here:

...the study has advantages over most of the research commonly cited as evidence that secondhand smoke causes lung cancer. "To our knowledge," the authors say, "this is the first study to examine both active and passive smoking in relation to lung cancer incidence in a complete prospective cohort of US women." The prospective design avoids a weakness of studies that start with lung cancer cases and "match" them to controls. "Many studies that showed the strongest links between secondhand smoke and lung cancer were case-control studies, which can suffer from recall bias," notes the JNCI article, since "people who develop a disease that might be related to passive smoking are more likely to recall being exposed to passive smoking."

The referenced study is from the Journal of the National Cancer Institute. Its key conclusions are (my emphasis):

A large prospective cohort study of more than 76,000 women confirmed a strong association between cigarette smoking and lung cancer but found no link between the disease and secondhand smoke.

To be honest the National Cancer institute does mention that there is a link between cancer and people living with smokers for over thirty years. Since they also say it's a dosage-related effect, I don't see that article as supporting such a strong answer.
– Sklivvz♦Jan 1 '14 at 19:00